As a method of printing plastic containers, cartons, plastic bags, boxes and envelopes, flexographic printing is widely used. As flexographic printing plates used in this flexographic printing, those formed by exposure, to light, of a composition for photosensitive flexographic printing plates which contains an elastomer, a polymerizable ethylenically unsaturated monomer and a photopolymerization initiator, are frequently used.
The composition for photosensitive flexographic printing plates is usually formed into a sheet and supplied as a multilayer sheet having a flexible sheet as a supporting body arranged on one side and a protective film arranged on the other side. By exposing the multilayer sheet to light at the side of the supporting body, the layer of the composition for photosensitive flexographic printing plates is cured to specific thickness. Then, the protective film is removed and a negative film is closely applied to the surface of the composition layer, and the negative film is irradiated with light. The part of the composition for photosensitive flexographic printing plates that is penetrated with light is cured, and an uncured part is removed by an organic solvent or an aqueous solvent, whereby a flexographic printing plate having a concave-convex structure is formed.
As for the elastomer used for producing a composition for photosensitive flexographic printing plates, a thermoplastic elastomer having excellent processability is widely used. In particular, as having sufficient rubber elasticity and flexibility and rebound resilience that are suitable for producing a flexographic printing plate, an aromatic vinyl-conjugated diene-aromatic vinyl block copolymer such as styrene-isoprene-styrene block copolymer (SIS) and styrene-butadiene-styrene block copolymer (SBS) is commonly used as an elastomer for producing a composition for photosensitive flexographic printing plates. In this connection, various studies have been made on the improvement of an aromatic vinyl-conjugated diene-aromatic vinyl block copolymer used for producing a composition for photosensitive flexographic printing plates.
In Patent Literature 1, for example, it is disclosed that a photocurable polymer composition, which is used for producing a flexographic printing plate, is obtained by using a mixture of SIS and a styrene-isoprene block copolymer (i.e., diblock). It is described that the photocurable polymer composition has excellent balance of various the physical properties that are required for a flexographic printing plate, such as flexibility for wrapping a printing cylinder and wear resistance for enduring repeated printing, etc.
Further, in Patent Literature 2, it is disclosed that a photocurable polymer composition for producing a flexographic printing plate is obtained by using a mixture of SIS and SBS. It is described that the photocurable polymer composition has high wear resistance and excellent flexibility without having excessive hardness. It is also described that the photocurable polymer composition does not exhibit anisotropy that is caused by molecular orientation occurring during melt-molding and thus it can avoid a bad influence on printing which may occur when a flexographic printing plate prepared with a material having anisotropy is used.
Further, in Patent Literature 3, it is disclosed that a block copolymer composition for flexographic printing plates is produced by using a composition containing a three-branched aromatic vinyl-conjugated diene block copolymer that is obtained by using a specific coupling agent. It is described that the block copolymer composition for flexographic printing plates shows excellent surface smoothness and excellent antiflowing properties (i.e., properties that are resistant to any deformation before curing by photosensitization) when a flexographic printing plate is prepared therewith.
However, even when the compositions described in Patent Literatures 1 to 3 are used, it remains difficult to obtain a flexographic printing plate having sufficient wear resistance while maintaining the rubber elasticity that is expected for the aromatic vinyl-conjugated diene-aromatic vinyl block copolymer. Specific reasons are as follows. It is known that mechanical strength and properties such as wear resistance of an aromatic vinyl-conjugated diene-aromatic vinyl block copolymer are improved by increasing the content ratio of an aromatic vinyl monomer unit included therein. However, there is a problem that, when the content ratio of an aromatic vinyl monomer unit is increased to the level at which wear resistance sufficient for use as a flexographic printing plate is obtained, the polymer becomes to lose rubber elasticity.